Internal oxidation processes under non-ideal conditions^†

Abstract

Wagner’s classical theory of internal oxidation is concerned with a single oxidant diffusing into an alloy to precipitate particles of a single compound which do not themselves affect significantly the diffusion process. This paper is concerned with some of the complexities which arise when this ideal description is inapplicable. The simultaneous precipitation of multiple chromium compounds in heat-resistant alloys exposed to two or more of oxygen, sulphur, carbon and nitrogen is shown to be in qualitative agreement with theoretical prediction: discrete precipitation zones form and widen according to parabolic kinetics.

When lamellare or acicular precipitates grow parallel to the oxidant diffusion direction, the aligned precipitate-matrix interfaces can provide rapid diffusion pathways, accelerating the reaction rates. The effect is demonstrated by molybdenum sulphide needles in Ni-Mo alloys, Al₂O₃ rods in austenitic and ferritic alloys, lamellar and Widmanstatten Cr₂N in nickel base alloys and lamellar Cr₂₃C₆ in both austenitic and ferritic alloys. In many cases, the formation of the aligned precipitates can be described as a cellular phase transformation at the reaction front.

Keywords:

• internal oxidation processes

Notes

† First published in the proceedings of High-Temperature Corrosion and Protection 2000